The invention relates to the field of continuous ink jet (CIJ) printers, and more particularly to a method and a device for regulating or adjusting the stimulation of the ink jet.
It makes it possible to obtain a robust operation and controlled printing quality despite the variability of the implementation conditions, identified by various parameters: environmental conditions (measured in particular by the temperature), deflection amplitude, nature of the ink . . . .
Deflected continuous ink jet printheads comprise functional means that are well known by those skilled in the art.
FIG. 1 diagrams such a printhead according to the prior art. This head essentially comprises the following functional means, described successively in the direction of progression of the jet:                a drop generator 1 containing electrically conducting ink, maintained under vacuum, by an ink circuit 7, and emitting at least one ink jet 11,        an individual charge electrode 4 for each ink jet,        an assembly formed by two deflection plates 2, 3 placed on either side of the trajectory of the jet and downstream of the charge electrode 4,        a gutter 20 for recovering ink from the jet not used for printing so it can be returned to the ink circuit and thus be recycled.        
The functionality of these different means is described below. The ink contained in the drop generator 1 escapes from at least one calibrated nozzle 10, thereby forming at least one ink jet 11. Under the action of a periodic stimulation device placed upstream of the nozzle (not shown), for example made up of a piezoelectric ceramic placed in the ink, the ink jet breaks off at regular temporal intervals, corresponding to the period of the stimulation signal, in a precise location of the jet downstream of the nozzle. This forced fragmentation of the ink jet is usually caused at a so-called “break-off” point 13 of the jet by the periodic vibrations of the stimulation device. The distance between the outlet of the nozzle and the so-called “break-off” point depends on the stimulation energy. Hereinafter, this size will be called the “break-off distance” or “break-off length,” and identified as BL. The stimulation energy is directly related to the amplitude of the electrical signal for controlling the ceramics.
At the location of this break-off point, the continuous jet turns into a line 11 of identical and regularly spaced drops of ink, at a temporal frequency identical to the frequency of the stimulation signal. For a given stimulation energy, any other parameter being stabilized moreover (in particular the viscosity of the ink), there is a precise (constant) phase relationship between the periodic stimulation signal and the break-off moment, which itself is periodic and has the same frequency as the stimulation signal.
This line of drops travels along a trajectory collinear to the ejection axis of the jet, which theoretically joins, by geometric construction, the center of the recovery gutter 20. The charge electrode 4, situated near the break-off point of the jet, is intended to selectively charge each of the drops formed at an electrical charge value that is predetermined for each drop. To that end, the ink being kept at a fixed electric potential in the drop generator, a voltage window with amplitude Vc, predetermined, is applied to the charge electrode. This window is generally different at each drop period. For the drop to be correctly charged, the moment at which the voltage is applied slightly precedes the fractionation of the jet, in order to take advantage of the electrical continuity of the jet and attract a given charge quantity at the end of the jet. The moment at which the charge voltage is applied is therefore synchronized with the method for fractionating the jet. The voltage is then maintained during the fractionation to stabilize the load until electrical insulation of the detached drop. The voltage still remains applied a little after the fractionation to take the hazards of the break-off moment into account.
The charge quantity taken on by the drop follows the relationship:Q=−K*Vc 
where K is a constant for the implementation conditions of the printer, which depends primarily on the permittivity of the medium, the width of the slit, and the volume of the drops. Hereinafter, a drop will be considered to be charged at Vc (e.g. 100 volts) and its charge will be −K*Vc volts (e.g. −K*100 volts).
The two deflection plates 2, 3 are brought to a fixed relative potential with a high value that produces an electrical field Ed substantially perpendicular to the trajectory of the drops. This field is capable of deflecting the electrically charged drops that engage between the plates, by an amplitude depending on the charge and the velocity of these drops. These deflected trajectories 12 escape the gutter 20 to impact the medium to be printed 30. The placement of the drops on the drop impact matrix to be printed on the medium is obtained by combining an individual deflection imparted to the drops of the jet with the relative movement between the head and the medium to be printed. These two deflection plates 2, 3 are generally planar.
The recovery gutter 20 comprises, at its inlet, an opening 21 whereof the cross section is the projection of its inlet surface on a plane perpendicular to the nominal axis of the non-deflected jet, placed just upstream of the contact with the gutter. This plane is called the inlet plane of the gutter. “Nominal axis of the non-deflected jet” refers to the theoretical axis of the jet when all of the subassemblies of the head are manufactured and placed relative to each other nominally once the head is assembled.
It is known that the control of the operation of a continuous jet printhead requires, in addition to the functional means described above, the implementation of a certain number of complementary means making it possible to master, on one hand, the deflection of the drops (which is determined in large part by the electrical charge and the velocity of the drops) and on the other hand, to monitor the proper operation of the recovery of the non-printed drops.
To best master the deflection of the drops for printing, the following conditions should be met.
The break-off process of the jet should be done stably and reliably, at a predetermined distance from the nozzle corresponding to the inside of the charge electrode.
Furthermore, the synchronization of the charge with the break-off moment is adjusted on the proper phase.
Lastly, the velocity of the jet is adjusted to a predetermined value, the best thing being to measure this value and make it subject to an instruction by acting on the pressure of the ink.
To that end, the printheads according to the prior art generally comprise a device for measuring a representative size of the charge taken on by the drops. This measuring device is arranged downstream of the charge electrode.
Thus, document EP 0 362 101 describes a device making it possible to detect the charge phase, measure the jet velocity, and know the distance between the nozzle and the break-off of the jet. It involves a single electrostatic sensor placed between the charge electrode and the deflection plates as well as the processing of the associated signal. The sensitive core of this sensor and the circulation space of the charged drops in front of this sensitive core are protected from electrostatic disruptions by electrostatic shielding. The exploitation of the obtained signal, upon passage of specifically charged drops, called test drops, the presence of which is sensed by their electrostatic influence on the sensitive core of the sensor, makes it possible to take very precise measurements of the charge level of these drops and to define the entry and exit moments from the sensor, so the transit time dT, of these drops in the detection area of the sensor. Knowing the effective length L of the space passed through, it is then possible to deduce the average velocity V=L/dT of the drops passing through the sensor.
Document EP 1 079 974 describes a device made up of two electrostatic sensors arranged in two relatively distant locations, close to and along the nominal trajectory of the jet. The level of the signal on one of the sensors provides information on the quantity of charges taken on by a test drop and the temporal shift between the signals of the two sensors makes it possible to obtain the velocity of the drop.
Document U.S. Pat. No. 4,636,809 describes a detection of the current produced by the flow, at the gutter, of the charges contributed by a succession of test drops. The amplitude of the current provides information on the average charge level of the drops, and the time between the charge of a group of drops at the charge electrode and the detection of the current produced when this group reaches the gutter makes it possible to calculate the velocity of the jet.
Knowing the velocity of the jet via one of the methods described above, it is possible to check the velocity of the jet by periodically measuring that velocity and making its value subject to a sign by acting on the pressure of the ink.
The method usually adopted to choose the synchronization moment of the charge relative to the break-off, and which makes it possible to satisfy the synchronization of the charge with the break-off moment, consists of proceeding with a succession of charge trials with charge moments (also called “phases”) differently distributed over a drop period, and for each phase, measuring the charge level taken on by the drop; this electric charge level being representative of the effectiveness of the charge process of the drops and therefore, the appropriateness of the charge synchronization. Certain phases produce a mediocre or even very poor charge synchronization, but in general, a certain number of phases make it possible to obtain a maximal charge.
The charge phase that will be used during printing will be chosen from the latter.
This technique is taught, for example, in EP 0 362 101. This document also describes a method also making it possible to know the precise moment of the charge of a test drop that corresponds to the break-off moment of the jet (to within a phase) and therefore, knowing the jet velocity Vj determined via one of the methods described above, to be able to deduce the time of flight Tv between the break-off of the test drop and its entry into the sensor.
Knowing, by construction, the distance D between the nozzle and the sensor inlet, the distance BL is deduced between the nozzle and the break-off of the jet: BL=D−Vj×Tv.
To obtain a break-off of the jet that can be used under good conditions, on one hand one verifies that the break-off is within the field of the charge electrode, therefore at a determined distance from the nozzle (break-off position); and on the other hand, one ensures that the break-off of the jet is done stably and reliably (break-off quality: which will be specified below). This is done through an optimal adjustment of the stimulation that occurs practically by acting on the stimulation energy.
In a known manner, the stimulation energy is controlled by the level VS of the periodic voltage signal applied to the stimulation device (piezoelectric).
A break-off is considered stable and reliable (good quality) when it makes it possible to guarantee an optimal charge of the drops in an operating field of the printer characterized in particular by a temperature range (conditioning the viscosity of the ink) for a given ink.
Concretely, just before the break-off, the drop 90 is connected by a tail 91 to the following drop 90′ being formed (see FIG. 2a). The shape of this tail determines the quality of the break. The shapes most characteristic of a problematic break-off are the following:                very fine tail 91 (see FIG. 2b), which risks breaking unstably (the surface tension cohesion forces become weak relative to the electrostatic forces). When a very significant electric field exists between two successive drops charged at very different values (case of a strong charge followed by a weak charge), a stress concentration effect phenomenon at the tail creates electrostatic forces such that particles of charged matter are pulled from the very fine tail of the highly charged drop and rejoin the weakly charged drop by transferring charges. As a result, the drops no longer have their nominal charge, the deflection is thereby disrupted, and the printing quality deteriorates.        tail having a lobe between two narrow portions (see FIG. 2c), which can break in two places and create a satellite 95 isolated from the drop, this satellite takes on part of the charges intended for the concerned drop:        if its velocity is faster than the jet (fast satellite), the satellite 95 and its charges will rejoin the concerned drop 93 before deflection and reconstitute a nominal situation without noticeable consequences for the printing quality,        if the velocity of the satellite is identical to that of the jet (infinite satellite) or does not rejoin the concerned drop before deflection thereof, it will be poorly charged and the satellites will be violently deflected at the risk of dirtying the printhead,        
if it rejoins the following drop 90 (case of a slow satellite) it will transfer, to the following drop 90, charges from the concerned drop 93 and thus disrupt the deflection.
The shape of the break-off, aside from the rheological characteristics of the ink, is related to the stimulation level (excitation intensity). In general, the break-off shape changes, when the excitation increases, to go from a break-off with slow, then infinite, then fast satellites (under-stimulation) to a break-off without satellites whereof the shape of the tail evolves, then the break-off returns to a slow satellite regime (over-stimulation). At the same time, the position of the break evolves following the curve of FIG. 3. The latter shows the profile of the characteristic f yielding the Break-off distance BL as a function of the Stimulation voltage VS (BL=f (VS)).
When the stimulation excitation increases (from a low value), the nozzle/break-off distance (BL), which starts from a high value (natural break-off of the jet), decreases and goes through a minimum called “turning point” (Pr) corresponding to an excitation voltage VPr and a break-off distance DPr, then elongates again. The shape and the actual position of this curve depend on several parameters, in particular characteristics of the drop generator, the nature of the ink, and the temperature. The printhead is designed so that the functional part of this curve is located, at least in part, in the field of the charge electrode despite the variability of the mentioned parameters. On the other hand, there is a functional zone related to the break-off quality in which the printing is satisfactory (the charge of the drops is correct).
The intersection of the correctly positioned zone and the break-off quality functional zone corresponds to the operational stimulation range, which is characterized by an entry point (Pe) on the left corresponding to a piezoelectric excitation voltage VPe and a break-off distance DPe, and an exit point (Ps) on the right, corresponding to a piezoelectric excitation voltage VPs and a break-off distance DPs as indicated in FIG. 3.
In certain techniques of the prior art, the position of the operational stimulation range is estimated relative to the point where the satellites are infinite and/or at the turning point, these two characteristic points being detected indirectly, but the actual range is not known (U.S. Pat. No. 5,196,860, U.S. Pat. No. 4,631,549).
One significant difficulty is determining the optimal operating point (Pf in FIG. 3) in the stimulation range, i.e. the optimal stimulation level (VPf), to obtain nominal printing under given use conditions (type of ink, average temperature, . . . ) taking into account the variability of the parameters during the usage session of the printer (in fact, between 2 stimulation adjustments). The break-off distance DPf of the operating point is always greater than or equal to that of the turning point DPr.
The positioning of the optimal operating point Pf is generally done empirically, in the vicinity of the turning point Pr, rather towards its left on the curve or for a slightly lower excitation, which corresponds to a slight under-stimulation.
One of the known methods for determining the optimal operating point involves referring to the curve BL=f (VS) and positioning the operating point relative to the shape of the curve, represented by its drift, near the turning point:                U.S. Pat. No. 5,481,288 discloses the fact that the optimal charge synchronization phase depends on the position of the break-off modulo the number of phases defined per drop period. When the nozzle/break-off distance evolves, the phase rolling (velocity and direction of evolution of the phases) is representative of the drift of the curve BL=f (VS). The zone of the turning point is identified when the drift passes below a certain threshold and the operating point is positioned in that zone, following an empirical law established experimentally,        in document WO 2009/061899 the slope of the curve BL=f (VS) is used directly to determine the optimal operating point. The curve BL=f (VS) being determined, the operating point is positioned where the slope of the curve has a given value, established experimentally. A negative value of the slope places this point to the left of the turning point, and the lower the absolute value, the closer the operating point comes to the turning point. Here the determination of the break-off distance is done in a manner similar to that described in EP 0 362 101 already cited above.        
The methods for determining the operating point as described above are not fully satisfactory because the measurements done do not make it possible to characterize the break-off quality and therefore its robustness relative, in particular, to the high charges. Indeed, these measurements are based on the determination of the best charge phase to deduce BL; these measurements being done by very slightly charging the drops used for the test.
Another method for determining the operating point is taught in document EP 0 744 292. It consists, for each excitation level of the stimulation scanning, of emitting, repetitively, sequences of drops comprising a charged test drop, preceded and followed by at least one uncharged drop (guard drops). The test drops are then spatially separated from the guard drops by deflection, to be oriented towards a sensor yielding a size representative of the average charge of the test drops (only). The test drops being charged at a maximum useful value, if the charge process is optimal (case of a break-off exploitable under those conditions), the sensor will detect a quantity of maximum charges on the test drops. If charges are transferred from the test drop towards the following guard drop (due to the presence of satellites having become slow), the sensor will detect a smaller quantity of residual charges on the test drops. At the end of the stimulation scanning one can identify the operational stimulation range that corresponds to the zone where the quantity of charges taken on by the test drops is maximal.
This method improves the preceding one because the positioning of the operating point, placed empirically in that range, takes the break-off quality present under the test conditions into account. Indeed, the test is done under conditions where strong charges are used.
This solution does, however, pose the following problems.
First, the test and guard drops must be separated, as the usable sensors (with a reasonable design complexity and production cost) cannot discriminate, in a same line of drops, between a situation where the charge of the test drop alone is optimal and a situation where the same charge is distributed over two successive drops in the event of a charge transfer, because the average number of charges seen by the sensor remains unchanged in both situations.
Moreover, the test drops must be deflected to be detected, but also recovered and returned to the ink circuit because the test operation is generally done outside printing; it is therefore necessary to implement a second gutter provided with a second sensor. The solution proposed in EP 0 744 292 requires a specific deflection electrode for that function. This entire dual-gutter system and dual-deflection system is complex and costly.
Moreover, during scanning of the stimulation excitation, the break-off goes through states where the risk of the appearance of infinite satellites exists. These charged satellites will be violently deflected by the deflection field due to their low mass and will dirty the elements of the head (in particular the deflection plates, at the risk of making the deflection field generator switch), which will require a maintenance operation.
Moreover, a repetitive sequence of a set of drops whereof a charged drop preceded and followed by an uncharged drop does not represent the worst case of using a CIJ printer where one can find successions of highly charged drops creating electrostatic conditions that are more restrictive regarding the transfer of charges.
The main drawbacks of the prior art can be summarized as follows:
The methods based on the detection of the turning point and/or the point where the satellites are infinite does not take into account the break-off quality, with the result that the operating point can be chosen outside the functional stimulation range.
The stimulation range determined at a low charge voltage and a nominal temperature is not that which guarantees an optimal printing quality at a high charge voltage and in the operating temperature operational range.
The curve BL=f (VS) determined by the methods of the prior art can be only partial, the turning point being outside the operational field of the detecting means used. Choosing the operating point relative to the operating point is then not possible.
In the method measuring the actual charge of the test drops, it is necessary to spatially separate the test and guard drops, which leads to a complex and costly system.
The repetitive sequences of an assembly formed by a charged drop preceded and followed by a guard drop do not take into account the reality where, in certain cases, a succession of drops can all be highly charged and create a more restrictive electrostatic environment than the test situation.